1. Field of the Invention
The present invention relates to a material sensitive to tactile stress (a tactile-sensitive material) utilizing micro coils with a spiral shape, and further to a tactile sensor formed with the tactile-sensitive material. The present invention further relates to a manufacturing method of the tactile-sensitive material.
2. Description of the Related Art
A tactile sense, which is one of the five senses of human, is a sense to receive mechanical stimulations generated from the contact with outer objects; however being broadly interpreted, it includes a skin sensation such as pressure sense generated from the contact, tough feeling and so on.
In recent years, many researches have been made to embody the tactile sense, which has a great deal of information, as artificial sensors. Such tactile sensors could be applied to various fields such as, for example, a robot having a skin sensation comparable with or exceeding that of human.
As examples of existing and simple tactile sensors, taken are an electric capacity type sensor, a piezoelectric type sensor, and an optical type sensor. However, these sensors simply measure applied stress numerically, generally with low sensitivity, and are difficult to be miniaturized.
Whereas, currently, a tactile sensor using carbon micro coils (CMC) is proposed and gathers attention. The CMC is a coil having a coil diameter from several hundreds nanometers to several micrometers and a helical shape, and is a new material that is expected to be applied to various fields such as an electromagnetic-wave-absorbing material, a microwave-absorbing material, a bioactive agent and so on. The structure and manufacturing method of the CMC is described in detail, for example, in Japanese Patent Publication No. 2006-321716A.
The CMC has an amorphous structure of carbon and a feature with high elasticity; therefore, it easily expands and contracts in response to even minute stress. When the CMC expands or contracts, also varies inductance (L), capacitance (C) and resistance (R) that are electric properties of the CMC. As an example of utilizing the anomalous characteristics of the CMC, Japanese Patent Publication No. 2007-121238A describes that the CMC is applied to a screen for infrared emission of an electric-wave-visibility apparatus, and further, Japanese Patent Publication No. 2006-184098A describes that the CMC is applied to conductive pressure-sensitive rubber of a pressure-sensitive sensor. However, attention is especially attracted by the above-described advanced tactile sensor formed with combined materials in which CMCs are dispersed uniformly in an elastic resin, as described, for example, in Japanese Patent Publication No. 2005-49332A and Japanese Patent Publication No. 2005-291927A.
Here, FIG. 1 shows the tactile sensor utilizing the CMCs described in Japanese Patent Publication No. 2005-49332A.
In a tactile sensor 11 shown in FIG. 1, CMCs 13 are dispersed randomly in solid medium 12, and a pair of electrodes 14 is provided respectively on the upper and lower surfaces of the medium. The dispersed CMCs 13 coupled electromagnetically through the medium 12 constitute a LCR resonance circuit. When a tactile stress, which is a mechanical stimulation generated from the contact with an outer object, is applied to a part or the whole of the tactile sensor 11 under the condition that an alternate current is applied to the LCR resonance circuit through the electrodes 14, an integrated variation in the L component, the C component and the R component occurs, which causes the variation in voltage or the like of the LCR resonance circuit. Thus, the tactile stress can be sensed by detecting the variation through the electrodes 14.
Further, the forming method of the medium 12 with the dispersed CMCs 13 is disclosed as follows: First, the predetermined amount of CMC is added into a base resin of silicon, and then, a hardening agent is further added to prepare a mixture. Then, the mixture is agitated to disperse the CMCs. After that, a mold is filled with the agitated mixture; then, by hardening the mixture, the medium 12 with the dispersed CMCs 13 is obtained.
In the tactile sensor formed by using the just-described method, the CMCs have rather random orientation in the resin. That is to say, the forming method cannot provide a controlled three-dimensional orientation of the CMCs in the resin. Further, as the sensor is miniaturized toward the order of the coil length, an unintended deviation in the orientation of the CMCs inevitably occurs. Originally, it is quite different to completely randomly orient coils having shapes such as the CMC in a limited space. Such a deviation causes the variation in characteristics of the sensor, further may cause the decrease in sensitivity of the sensor in the case of its miniaturization. As the measure for this problem, Japanese Patent Publication No. 2005-49332A describes the technique in which the CMCs 13 are oriented toward the direction parallel to lines of the magnetic field which is applied to the medium 12 with the dispersed CMCs 13. However, the degree of the orientation has a certain limitation even if a magnetic field with fairly substantial intensity is applied to such a coil as the CMC.
Further, the synthesized CMCs have various shapes, various coil diameters, whole lengths and so on according to the synthesizing condition, and the synthesized batch of the CMCs has a certain distribution of them. Therefore, there are limits of desired uniformities of the shape and the size of the CMCs. Even such non-uniform CMCs are not so easy to be produced.